1. Field of the Invention
The invention concerns a circuit arrangement for the protection of a component that is series-connected to a voltage source with an inductor, particularly a switching semiconductor, against an overcurrent caused by magnetic saturation of the inductor.
2. Description of the Prior Art
Application examples of switching semiconductors in connection with inductors are, e.g. the single ended throughput current transformer, the push-pull throughput current transformer, or DC motors controlled by switching semiconductors. There are numerous other applications in which a component that is series-connected to an inductor can be protected against the overcurrent that occurs in the case of magnetic saturation of the inductor. The invention is described in its application to a throughput current transformer circuit below, but for the above reasons it is not intended to be limited to this application.
The magnetic saturation of the core material of an inductor may have different causes. One reason for magnetic saturation may be an excessive operating voltage to which the inductor and the component that is series-connected with it, and is to be protected, are connected. Magnetic saturation can also result from a switch in series with the inductor, for example the already mentioned switching semiconductor, remaining cut in too long, so that the magnetization current in the inductor exceeds the saturation value. This error may occur in current transformer circuits, for example, in which a switching semiconductor is series-connected with the primary winding of a transducer. In single ended throughput current transformers insufficient demagnetization of the inductor of the transducer may also occur, prior to the renewed cut-in of the switching semiconductor, which also causes magnetic saturation of the transducer. This state may correspondingly occur in a push-pull throughput current transformer, if the transducer was insufficiently backmagnetized prior to each cut-in of the switching semiconductor.
The above described cases result from malfunctions of the voltage supply or the control circuits within the described current transformer circuits. In a push-pull throughput current transformer one must principally expect saturation upon the first cut-in, unless the energizing period is additionally reduced for the cut-in. In such circuits an asymmetry of the switching periods of the switching semiconductors being used and the related DC bias on the transducer may cause magnetic saturation.
The magnetic saturation of the core material of an inductor brings about a severe drop in inductance, accompanied by a steep rise in current over time. It is now possible to provide special protective circuits for overcurrent cutoff, which open the circuit when the current corresponding to the point of magnetic saturation is reached. Such overcurrent cutoffs take place with an unavoidable delay, however, so that the component to be protected is still exposed to a relatively high excess current during the delay time. Such a delay is particularly detrimental in the case of series connections of inductors and switching semiconductors, since these have a long switching delay, corresponding to their rather long storage time, on the order of about 2 to 3 microseconds, during which they can be destroyed by the described overcurrent, even if a special current guard circuit causes them to be disconnected when the inductor with which they are series-connected reaches the saturation state.